For the purpose of realizing higher output power and lower power consumption, double-stage supercharging systems have been conventionally proposed for internal-combustion engines. For example, PTL 1 discloses a double-stage supercharging system configured such that a cooler is interposed between a low-pressure turbocharger and a high-pressure turbocharger and that a flow of air compressed at the low-pressure turbocharger is cooled at the cooler and then, is supplied to the high-pressure turbocharger.